Hand held, pneumatically operated fastener driving tools are old and well known and enjoy widespread use in many areas, such as, the construction field. In general, such tools comprise a gun-like body having a piston-cylinder driver means connected to a pneumatic source, a drive track into which are fed individual fasteners from an attached magazine, and a nosepiece for contacting the workpiece so that a fastener can be driven thereinto. The tools invariably assume the shape of a form of handgun having a handle and an associated trigger for actuating the driver. Those skilled in the art will appreciate that the fasteners are driven from such tools with relatively high and explosive forces which renders dangerous improper or accidental use of the tools. Accordingly, there have been numerous prior efforts at providing mechanisms for preventing such inadvertent or accidental uses from occurring.
A common and well known expedient is the use of a movable nosepiece which normally de-activates the tool and prevents firing until the nosepiece is pressed against a workpiece. Representative examples of such nosepiece arrangements may be seen in U.S. Pat. Nos. 3,194,324; 3,198,412; and 3,519,186. In each of those prior art devices, the movable nosepiece comprises an element of the main housing and is linked directly to the trigger means for preventing unintentional firing. Pat. No. 3,198,412 also teaches a means for inactivating the trigger means when the magazine is empty or almost empty of staples. Another example of the movable nosepiece feature is shown in U.S. Pat. No. 2,979,725, where the nosepiece and actuating member are carried by the fastener magazine which is removable from the main housing. In the latter device, the movable actuating member controls the pneumatic valve arrangement rather than acting upon the trigger means itself.
Yet there exists a need for a mechanism that positively insures against firing when the fastener magazine is empty or has been detached from the housing. This need exists because of the not unlikely possibility that a single fastener may remain unobserved in the drive track even when the magazine has been removed or is seen to be empty of fasteners. Moreover, it is desirable that such additional mechanism act to de-activate the trigger mechanism rather than the pneumatics because of the natural tendency to carry a gun-like device by its handle so that one or more fingers automatically rest on the trigger means.